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How to Fix a Petrol Lawnmower That Won't Start

Now if you’ve had problems starting your petrol lawnmower, you’ll know just how frustrating that can be! In this video we are going to run through some of the basic checks that can be done to make sure your mower starts first time every time.

If you are unsure where to start there’s no need to panic. We’ll talk you through how to successfully complete these checks without damaging your lawnmower.

This video shows an example on how to remove or replace the part on a typical machine, some models may be different but the procedure should be similar.

What You Will Need

Step 1 - Safety First

Safety First! Please make sure that the mower cannot start by disconnecting the spark plug lead before starting your repair.

Check 1 - The Fuel

A standard four stroke engine takes ordinary unleaded petrol, now the thing about petrol is it degrades over time especially in a warm environment it can go off. So if you’ve left petrol in your mower over the winter, the best thing to do is to drain that off first and put some fresh petrol in there.

The first thing to drain is the main fuel reservoir, just by uncoupling the fuel line and proceed to drain it into a bowl.

The second thing is to drain the fuel off the carburettor. You will see on the carburettor that there is a little release valve to drain it away, and do make sure that you then have fresh petrol.

Check 2 - The Oil

The dipstick on the Qualcast model machine in the video is at the back. Just take it out and give it a quick wipe and then pop it back in before removing and reading the level. In this instance it shows as being measured at just over half full - and that’s fine.

If you do need to top it up though, then top up at the front and make sure you’re using proper lawnmower engine oil. Obviously if yours is a two stroke then the two stroke oil must be mixed directly into your fuel.

Check 3 - The Spark Plug

On this lawnmower model it features a standard 14mm lawnmower spark plug, and we are using a 21mm socket which is specifically for spark plugs. When removing the spark plug make sure that you don’t damage the porcelain or the top of the engine.

Spark plugs do have a limited life, and this is typically about 150 hours for this type of spark plug. As you can see on this particular spark plug on this lawnmower, there’s a spare bit of sooting, and a fair bit of carbon on the electrodes too.

If it does not look too bad, then you can choose to clean it with a wire brush and also just get a bit of fine sandpaper between the two electrodes, which will help to make sure that it’s nice and clean.

Alternatively you can choose to replace the spark plug with a new one - but you will need to make sure you’re getting the right spark plug for your particular mower (Check out the eSpares website for our range of spark plugs).

Finish fitting the spark plug back on your mower by tightening it up, but careful not to do it too tight.

Check 4 - The Belt

Remove the belt housing cover by simply undoing the screws that are holding it. At this point just engage the rear roller, to make sure that it’s tightening the belt when it’s in the “on” position.

Likewise, then engage the cutting blades to check that it’s putting some tension in the belt. Follow this by then just having a look at the belts, making sure they’re not fraying or that there is not any sign of cracking, damage or wear.

Check 5 - The Gears

On this particular model there are actually three nylon gears. These gears can actually break quite easily, and you can see an alternative video on eSpares where it shows how to actually replace them. However, the gears on this mower look fine.

Check 6 - The Throttle

Ensure that the mower is in the fully “off” position, and you should be able to see that just cuts the power to the spark plug. As you then open the throttle, you should be able to see it’s just opening the carburettor. If that needs any adjustment, there is a little screw that just changes the position at which it opens the carb. We suggest not to adjust the carburetor until the motor has been running for five minutes and is warm.

Let’s Try To Start The Lawnmower Now!

After you have completed all the checks that have been outlined above, try starting your lawnmower to see if it will now start correctly. Push the throttle all the way back, and all the way open. If you find that your mower is still not starting, there could be an issue with the carburettor.

Check 7 - The Carburettor

Before you open up the carb, you need to disconnect the fuel line and drain the fuel in the tank down into the cam. With the fuel line out of the way, get the air filter out of the way as well, and you can proceed to drain the remaining fuel that is just sat in the carb (you can do it on the release valve). Get as much paper towel under the valve as possible, so that any drained fuel can be soaked up.

Open the nut on the bottom of the carburettor now, and so you can see what it has got inside. As you undo the bottom screw, you will notice that the float chamber comes away, which you can just pop to one side for the moment.

You will now be shown the float itself, so proceed to push through the hinge pin and then pull the whole of the float mechanism away. Looking at this particular float, there are two things to be concerned about. Firstly the float valve piston, which sits on top of the float and is what rises and falls with the valve to let fuel into the float chamber.

The bottom screw that you had taken out has fuel jets in there, and you need to proceed to clean both of these two things thoroughly. For the jets, you can choose to run a piece of fine wire through them, not forgetting to run one through the middle just to make sure that there is no debris in there. Give the piston valve a good clean with compressed air, or alternatively you can opt to get a special carburettor cleaner.

It’s also a good idea to spray that cleaner or the compressed air around the rest of the carburettor. At this stage just check the gasket seal as well, and if it’s looking bit of a mess then it’s probably a good idea to replace the seal.

Ok checks now complete, it’s time to put the carburettor back together. You can see on the float chamber that there’s a shallow and a deep end. Make sure you get the shallow end where the hinge of the float goes and then just put the screw back in. Don’t forget to reconnect the fuel line and, of course, put the air filter back on. There we go, you just need to top up with fresh fuel and you’re ready to go!

Make Sure You Maintain The Best Performance From Your Lawnmower

Here at eSpares we have a wide range of care and maintenance products that could be of benefit to your lawnmower - including oils and cleaning sprays that will help to give the machine the TLC that it needs to function efficiently. So to help maintain the working performance of your mower, you can choose to check out the range of various oils and sprays we have on the eSpares website.

Need To Locate The Correct Parts For Your Lawnmower?

Here at eSpares, we have a wide-range of lawnmower spares from blades, belts and oils. You can also choose to view our full range of replacement lawnmower spares - so you can soon be on your way to repairing it yourself.


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